Description:The cliffs which are located below the 1412m high Montaña del Horno are of great interest geologically in that the rocks forming the cliffs were deposited inside of the Miocene, 1000m deep, 18 x 28 km wide Caldera de Tejeda, between 14.1 and 13 million years ago, eventually completely refilling this caldera. Since then, the area has not been affected by any volcanic activity and the erosion over 12 million years has largely eroded the outer rock formations revealing the intra caldera materials and part of the caldera wall, forming these incredible cliffs. The green coloured layers are composed of hydrothermally altered pumice deposits erupted at 13.29 million years, that were deposited on the inside of the outer margins of the caldera, very close to the top of the caldera wall and are overlain here by at least 5 thicker layers of red hydrothermally altered rhyolitic ignimbrites dated as 13.16 my.
The hydrothermal alteration was caused by the heat escaping from a newly emplaced, shallow magma chamber, through fissures within these very porous, water saturated pyroclastic and epiclastic deposits that had filled in the caldera to the brim, resulting in widespread steam fumaroles and hyrothermal vents.
As the resultant chemical and moisture rich, 200 -250 degree centigrade gas came into contact with the pumice and ignimbrite deposits close to the outer margins of the caldera, it altered the composition of the minerals contained within these rocks, changing their colour and composition.